The viscoelastic properties of a material are described by physical parameters such as the viscous modulus (G″), the elastic modulus (G′), or else by the relationship
                    G        ″                    G        ′              =          Tg      ⁢                          ⁢      δ        ,which is the tangent of the angle of offset between stress and deformation when the material is subjected to sinusoidal stress, and which makes it possible inter alia to characterize the phenomena of dissipation inside the material.
There is a wide range of means which make it possible to measure these physical characteristics. The most widespread means are oscillating rheometer, in which the sample to be evaluated is held between two plates, rotatable one respectively to the other. The values of the viscous and elastic modulus results from the measurement of the efforts exerted by the sample upon the axis of rotation, once the mobile plate is oscillating within a slight angular value. These means are known, as example, from publication U.S. Pat. No. 2,752,778, or from publication WO 02/42739. One other means, the physical principles of which have been described by Gent in the Journal Apply of Physics (1960, 11, 165) or by Maxwell and R P Chartoff in the review Trans. Soc. Rheol (1965, 9, 41), is known by the name of orthogonal rheometer. The physical laws of such a rheometer have been developed by way of example by C. W. Macosko and W. M. Davis in the manual dedicated to rheometry which bears the title Rheometry Acta (1974, 13, 814).
An orthogonal rheometer, a block diagram of which is shown in FIGS. 1 and 2, comprises two rotating plates 10 and 20, the planes of which are parallel to one another, and which are spaced apart by a given distance e. The sample E to be measured, of cross section S, is placed between the two plates. The axes of rotation of the two plates, respectively a1 a1′ and a2 a2′, are not collinear, but rather are offset by a distance d in a direction XX′ which is perpendicular to said axes of rotation and parallel to the plane of the plates.
The orthogonal rheometer of the prior art, which is described in the aforementioned work by Maxwell and Chartoff, comprises a driving motor which is able to drive the upper plate in rotation at a constant speed ω, the other plate being held by a shaft with the lowest possible friction resistance. This shaft is itself driven in rotation via the sample E, at a speed of rotation equal to ω.
The lateral forces exerted by the sample on the lower plate in the direction XX′ and in the direction YY′, which is perpendicular to the direction XX′ and to the axes of rotation a1 a1′ and a2 a2′, i.e. respectively Fx and Fy, are measured by suitable means and make it possible to calculate the values of G′ and of G″ at a stress frequency equal to ω.
By considering the value
      γ    =          d      e        ,the following results are obtained:
            G      ′        =          Fx              S        ⁢                                  ⁢        γ                        G      ″        =          Fy              S        ⁢                                  ⁢        γ                        Tg      ⁢                          ⁢      δ        =          Fy      Fx      These equations are valid when the value of γ is sufficiently small, and when the effects associated with the inertia of the plates is ignored.
It will be noted that one of the known advantages of this type of rheometer is that it makes it possible to measure a sample having a cross section of any shape, provided that the value of this cross section is known at the time of measurement.
The publication U.S. Pat. No. 4,095,461 describes an orthogonal rheometer which is based on these principles, in which the upper plate is driven in rotation by a motor mounted on a fixed chassis, and in which the lower plate is mounted on a shaft which has a very low resistance to rotation, and on which the lateral forces Fx and Fy are measured.
The lower shaft is mounted on a platform which is able to move with respect to a fixed chassis, so as to make it possible to move the axis of the lower plate away from the axis of the upper plate by a desired value d in the direction XX′.
However, the type of construction described in the publications mentioned above by way of reference gives rise to stresses which are prejudicial to the quality of the measurement. This is because, with this type of mounting, the sample drives the lower plate in rotation, and a friction torque is created between the movable platform and the lower support, which is associated with the braking torque generated by the bearings of the lower plate.
Moreover, the vertical load Fz which is applied so as to keep the sample E between the faces of the upper plate and the lower plate creates a friction torque which is felt in the sample E, which is then subjected to a parasitic torsional stress. This results in a change in the value of the forces Fx and Fy, which takes the form of an alternating sinusoidal signal (of the same frequency as the rotation frequency ω). Analysis of the measurement signal then requires the use of sensors with a very high pass-band, of a filtering means, and of suitable data processing software.